I had the same problem. There is a large gear at the middle rear of the clock. Riding on top of this gear is a dark metal four-fingered gear. If you look in the back while you manually rotate the minute hand clockwise, you should see the dark gear move in sync with the minute hand. This is a compression fit component that can loosen with use. All four fingers must be firmly pressing against the larger gear upon which it is mounted. Make sure none of the fingers are bent. If the center mounting hole is worn, it can sometimes be fixed using a small hammer and anvil. Too much hammer and you may have to bore out the hole a little. It’s important that this gear fit tightly to the shaft so as to keep its fingers pressed tightly against the other gear.
Here is a quick check you can perform to see if this gear may be loose. Remove the weight controlling the time movement. Manually rotate the minute hand clockwise. If you find little or no resistance to the movement, then this may be the problem.
Jack Spratt, May 2011